The present invention relates to data storage testing and recovery systems, and more particularly, this invention relates to systems and methods to test and/or recover sensors with Electrostatic Discharge (ESD) and other magnetic damage.
ESD damage to Giant Magnetoresistive (GMR) sensors is a major source of yield loss for magnetic GMR read sensors used in tape and hard disk storage drives as well as other applications. The damage mechanism which has one of the lowest current/voltage thresholds for damage is the pinned layer reversal. A pinned layer may be present in a magnetic sensor to act as a reference to the free layer and/or to stabilize other layers. Pinned layer reversal may occur at current/voltage levels about half those required for permanent damage. When diodes are connected in parallel with the GMR sensor, the sensors are damaged in the same manner as without diodes, except the damage thresholds are increased by factors of 5 to 10 or more.
The standard method of recovering a GMR sensor with a pinned layer reversal is to apply a bias current to the sensor of in the appropriate bias direction to favor normal pinned layer orientations and with a high enough magnitude and of an appropriate pulse width (time duration) to cause the reversal to the proper orientation while not causing damage to the sensor.
Pulse generators have been used to achieve the appropriate results. The problem with prior approaches have been the need for expensive pulse generators. Another problem with programmable pulse generators is that short time duration pulse generators have limited current levels, usually 10 to 100 mA for a 10 ns pulse width, and even less for shorter pulse durations. When diodes protection is applied to the sensors, the currents required to cause a pinned layer reversal for a pulse width of <10 ns can be of the order of 1 A or more. Standard, inexpensive, programmable pulse generators are not designed for generating currents high enough to recover the pinned layer reversal using short time pulses, and require expensive special equipment. One approach used write drivers in a hard disk drive (HDD) to pulse the head for recovery of heads without diode protection. Since write drivers typically are supplied by about 5 to 10 V maximum, recovering a diode protected sensor will probably not be possible, since the voltages required can be 50 V or more. Furthermore, in the case of tape drives, where the number of sensors is 36 or more in extant drives, cost of the circuitry to switch the sensors between the pulse and the normal operation is significant, since each drive would require the relay switches for each sensor.
Another issue with recovery is the desire to know whether the sensor is recovered. To measure the recovery, one typically measures the sensor response to an applied external magnetic field, such as an electromagnet or by reading data written on magnetic media. These methods are impractical and expensive. Reading data from magnetic media requires mounting the head onto a magnetic tester to read the media, which is time consuming. Applying an external magnetic field on the sensor will not work with head which has a magnetically actuated head. For example, tape heads contain magnetic actuators which could be damaged by magnetic torques if placed in a strong homogeneous magnetic field.